Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy to light. Improvements in these devices have resulted in their use in light fixtures designed to replace conventional incandescent and fluorescent light sources. The LEDs have significantly longer lifetimes and, in some cases, significantly higher efficiency for converting electric energy to light.
One class of LED-based light sources that are designed to replace conventional white light sources utilize a “white LED” in combination with a red LED. The white LED is typically constructed from a blue LED that is covered with a phosphor layer that converts a portion of the blue light to yellow light. If the ratio of blue to yellow light in the output spectrum is correct, the light appears to be “white” to a human observer. For some applications, the “color temperature” of the resulting light source is too high. In addition, the output spectrum is less than ideal in terms of its color-rendering index. Accordingly, a red LED can be incorporated with the white LED to fill in the spectrum at long wavelengths and to shift the perceived color of the light source to a lower color temperature. For the purposes of this discussion, a light source constructed from two different types of LEDs will be referred to as a compound light source.
One problem with such compound LED light sources is dependence of the output spectrum on the temperature of the LEDs. The fraction of the electrical power dissipated in the LED that is converted to light will be referred to as the electrical conversion efficiency in the following discussion. The electrical conversion efficiency of an LED typically decreases with increasing temperature. For any given increase in operating temperature, the amount of the decrease depends on the particular type of LED. The electrical conversion efficiency of red LEDs decreases with temperature faster than the electrical conversion efficiency of blue LEDs. Hence, the ratio of red light to blue light in the above-described white light source changes with temperature. This leads to a color shift as the temperature of operation increases. In light sources that are designed to replace conventional incandescent or fluorescent sources, the LEDs often operate at temperatures that are significantly above ambient. The exact operating temperature depends on the specific light source fixture and the ambient temperature.
In addition to the change in electrical conversion efficiency with temperature, the output wavelength of the LEDs also shifts with temperature. For example, the wavelength of light from red LEDs increases with increasing temperature. This shift results in an additional shift in the color temperature of the light source. Accordingly, providing a compound light source that produces light of a predetermined color temperature at all operating temperatures presents engineering challenges.